1. Field of the Invention
The present invention relates to a method and apparatus for discriminating synchronous or asynchronous states of Viterbi decode data, by performing independent trace-back of the survival path of the received data at the Viterbi decoder.
2. Description of the Prior Art
A Viterbi decoder is a decoder which uses the Viterbi algorithm to decode the convolutional encoded data and is well known. The convolutional encoding and the Viterbi algorithm are used for correcting errors caused by transmitting and recoding data in digital communication. In Viterbi decoding the convolutional encoded data, it should be determined whether the decode data is synchronous or asynchronous. That is to say, any apparatus using the Viterbi decoding algorithm, for example, a high definition television, a digital satellite communication, a digital cable television, a hard disk drive and a floppy disk drive, should determine whether the received data is synchronous or asynchronous. In the prior art, there are two methods for discriminating synchronous or asynchronous states of the received data.
Referring to FIG. 1, first, the convolutional encoded data is decoded in the Viterbi decoder 1. Then, the output data of the Viterbi decoder 1 is convolutional reencoded by a convolutional encoder 3. The delayer 2 delays the received data, then a comparator 4 compares the delayed data with the convolutional reencoded data. If the convolutional reencoded data is identical to the delayed received data, there is no error in the transmission of data. If a small portion of the convolutional reencoded data is not identical to the delayed received data, there is an error in the transmission of data. If a large portion of the convolutional reencoded data is not identical to the delayed received data, the decoded data is not synchronized to the delayed received data. The method relating to FIG. 1 has a problem in which the difference between a small portion and a large portion of no identification is unclear.
In another method for determining synchronization, a minimum value of the path metric is selected at every cycle when the convolutional encoded data is Viterbi decoded. Then, the difference between the present minimum value and the next cycle minimum value is examined. If the difference is less than a threshold value which is determined experimently, it is determined to be in a synchronous state. If the difference is more than the threshold value, it is determined to be in an asynchronous state. In this case, it is difficult to implement the circuit for selecting the minimum value of the path metric. Also, it is difficult to decide the threshold value.
Therefore, it is an object of the present invention to provide a method and apparatus which determines synchronous or asynchronous of the Viterbi decoded data by the independent multiple trace-back of the transmission path of the convolutional encoded data.